Refactor epics split button in RelatedItemsTreeHeader component to use gitlab-ui

The following discussion from !17948 (merged) should be addressed:

  • @winh started a discussion:

    If you click the epic split button first and the new one afterwards, both are open:

    Screen_Shot_2019-10-14_at_19.44.21

    This is a problem in the existing split button, so I will create a follow-up issue to fix it.